1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device used in a liquid crystal display panel or a liquid crystal light valve, etc.
2. Description of the Prior Art
Today, display devices using liquid crystal (liquid crystal display devices) have been applied in various kinds of field such as finders of video cameras, pocket TVs, highly fine projector-type TVs, and personal computers, word processors or the like data display terminals. The development and commercialization of the above display devices are proceeded lively. Particularly, an active matrix type liquid crystal display device of all draws a passionate attention, because it ensures high quality of the image. The active matrix type is a contractive driving method of liquid crystal to the conventionally-employed simple matrix type. More specifically, in the active matrix driving method, each pixel electrode disposed on the matrix is provided with a switching element, to which is independently supplied an electric signal to control the optical characteristic of the liquid crystal through the switching element. Therefore, the active matrix driving method can maintain high contrast even at the time of a large-capacity display, without accompanying crosstalk, not alike the conventional simple matrix method.
Currently, a thin film transistor is mainly used for the switching element in the active matrix driving method. FIG. 2 is a schematic equivalent circuit diagram of an active matrix type liquid crystal device using a thin film transistor array according to the prior art.
As indicated in FIG. 5, a thin film transistor (TFT) 1 as a switching element provided for each pixel electrode supplies an electric signal of a signal line 3 to the subject pixel electrode when the transistor 1 is selected by a scanning line 2. The signal line 3 is electrically insulated from the pixel electrode when the thin film transistor 1 is not selected. When the scanning line 2 makes no selection, the pixel potential is held by a liquid crystal capacitor 5 formed between the pixel electrode and a counter electrode 4 confronting the pixel electrode via a liquid crystal layer. Therefore, it is generally practiced that a storage capacitor 6 is formed at the side of the TFT 1 parallel to the liquid crystal capacitor 5 via the pixel electrode and the insulating layer so as to improve the voltage retention efficiency of the pixel electrode when the scanning line 2 does not select the transistor. The stability of the pixel potential (voltage applied to the liquid crystal) at the non-selecting time is greatly dependent on the characteristics oft he liquid crystal layer and the insulating layer constituting the storage capacitor. In other words, if the liquid crystal layer has a low resistance, a current leak occurs due to the resistance of the liquid crystal layer at the non-selecting time of the scanning line 2, thereby decreasing the voltage between the pixel electrode and the counter electrode 4. As a result of this, the effective voltage to be fed to the liquid crystal is lowered, which causes the optical characteristic of the liquid crystal to be deteriorated. It is needless to say that the same phenomenon as above is brought about also in the case where the resistance of the TFT 1 as the switching element is low at the non-selecting time. Accordingly, the stability of the electric characteristic of the liquid crystal layer and the TFT 1, and the design of the storage capacitor, i.e., the voltage retention stability of the liquid crystal layer at the non-selecting time, etc. play an important role to improve the reliability of the active matrix type liquid crystal display device.
Meanwhile, it is well known that the optical characteristic of the liquid crystal is deteriorated with time as a direct current voltage is applied to the liquid crystal. Nevertheless, it cannot be avoided in the active matrix type liquid crystal device that a direct current voltage is applied between the scanning line 2 or signal line 3 and the counter electrode 4. A direct current voltage which is negative to the counter electrode 4 is applied to pixels arranged particularly in the vicinity of outgoing electrodes of the scanning lines, namely, in the periphery of the display screen almost all the time. If the liquid crystal panel is continuously driven for a long time at high temperatures according to the conventional active matrix driving method, the voltage retention characteristic of the liquid crystal is changed from the periphery, resulting in the luminance irregularity or color irregularity consequent to the change of the transmittance of the panel.